1. Field of the Invention
The present invention generally relates to a magnetic roller for use in a developing device provided in an image forming apparatus employing two-component developer, and more particularly, to a magnetic roller fixedly mounted in a developing sleeve confronting an electrostatic latent image support member.
2. Description of the Related Art
FIG. 1 depicts a conventional magnetic roller 20 fixedly mounted in a developing device provided in an image forming apparatus.
The magnetic roller 20 has an outer diameter of, for example, 24.5 mm and comprises a core 21 and a magnetic member 22 formed on the peripheral portion of the core 21 except opposite ends thereof. The magnetic member 22 includes a mixture of ferromagnetic powder, for example ferrite powder, and polymeric compound (resinous material). Magnetic poles N3, S1, N1, S2, and N2 are formed in this order along the periphery of the magnetic member 22. As shown in FIG. 2, these magnetic poles are formed in a manner in which magnetic poles of external magnets 24a-24e are opposed to the periphery of the roller 20 which has not been magnetized yet.
In the image forming apparatus, developer is held on a developing sleeve while magnetic carrier contained in the developer is prevented from adhering to an electrostatic latent image support member. For this purpose, a magnetic pole confronting the electrostatic latent image support member requires a magnetic force of approximately 1000 gauss.
Furthermore, there is an increasing demand for a small-sized magnetic roller in order to comply with the recent request for a small-sized light apparatus or a multicolor apparatus.
However, in the above-described arrangement in which magnetic poles S and N are alternately formed along the periphery of the magnetic roller 20, if the magnetic roller 20 is reduced to 15 mm in outer diameter, the main magnetic pole N1, which confronts the electrostatic latent image support member and exerts remarkable influence upon the development, cannot be magnetized over 500 gauss. The reason for this is that the magnetic flux density for magnetizing the main magnetic pole N1 is low during a magnetizing process for the magnetic roller 20.
Accordingly, when two-component developer consisting of toner and carrier is used along with a small-sized magnetic roller, the amount of carrier which will be shifted to the electrostatic latent image support member is increased. This is disadvantageous in that the image quality would be lowered and the surface of a fixing roller is liable to be deteriorated.